This invention relates to a method of making amorphous alloys having an increased band gap and devices made therefrom. The invention has its most important application in making improved photoresponsive alloys and devices having large band gaps at least in a portion thereof for specific photoresponsive applications including photoreceptive devices such as solar cells of a p-i-n or p-n junction type; photoconducting medium such as utilized in xerography; photodetecting devices and photodiodes including large area photodiode arrays.
Silicon is the basis of the huge crystalline semiconductor industry and is the material which has produced expensive high efficiency (18 percent) crystalline solar cells. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free germanium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was accomplished by diffusing into such crystalline material on the order of parts per million of donor (n) or acceptor (p) dopant materials introduced as substitutional impurities into the substantially pure crystalline materials, to increase their electrical conductivity and to control their being either of a p or n conduction type. The fabrication processes for making p-n junction crystals involve extremely complex, time consuming, and expensive procedures. Thus, these crystalline materials, useful in solar cells and current control devices, are produced under very carefully controlled conditions by growing individual single silicon or germanium crystals, and, when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitation caused by the size limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an impossible economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion. Further, crystalline silicone has an indirect optical edge which results in poor light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the single crystal material is replaced by polycrystalline silicon with cheaper production processes, the indirect optical edge is still maintained; hence the material thickness is not reduced. The polycrystalline material also involves the addition of grain boundaries and other problem defects.
An additional shortcoming of the crystalline material, for solar applications, is that the crystalline silicon band gap of about 1.1 eV inherently is below the optimum band gap of about 1.5 eV. The admixture of germanium, while possible, further narrows the band gap which further decreases the solar conversion efficiency.
In summary, crystal silicon devices have fixed parameters which are not variable as desired, require large amounts of material, are only producible in relatively small areas and are expensive and time consuming to produce. Devices manufactured with amorphous silicon can eliminate these crystal silicon disadvantages. Amorphous silicon has an optical absorption edge having properties similar to a direct gap semiconductor and only a material thickness of one micron or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline silicon. Further, amorphous silicon can be made faster, easier and in larger areas than can crystal silicon.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of which can encompass relatively large areas, if desired, limited only by the size of the deposition equipment, and which could be readily doped to form p-type and n-type materials where p-n junction devices are to be made therefrom equivalent to those produced by their crystalline counterparts. For many years such work was substantially unproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordinated and were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for making p-n junctions for solar cell and current control device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon and germanium, W. E. Spear and P. G. LeComber of Carnegie Laboratory of Physics, University of Dundee, in Dundee, Scotland, did some work on "Substitutional Doping of Amorphous Silicon," as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of reducing the localized states in the energy gap in amorphous silicon or germanium to make the same approximate more closely intrinsic crystalline silicon or germanium and of substitutionally doping the amorphous materials with suitable classic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein a gas of silane (SiH.sub.4) was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and deposited on a substrate at a substrate temperature of about 500.degree.-600.degree. K. (227.degree.-327.degree. C.). The material so deposited on the substrate was an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material a gas of phosphene (PH.sub.3) for n-type conduction or a gas of diborane (B.sub.2 H.sub.6) for p-type conduction was premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The gaseous concentration of the dopants used was between about 5.times.10.sup.-6 and 10.sup.-2 parts per volume. The material so deposited including supposedly substitutional phosphorous or boron dopant was shown to be extrinsic and of n or p conduction type.
While it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum temperature with many of the dangling bonds of the silicon during the glow discharge deposition to substantially reduce the density of the localized states in the energy gap toward the end of making the amorphous material approximate more nearly the corresponding crystalline material.
In working with a similar method of glow discharge fabricated amorphous silicon solar cells utilizing silane, D. E. Carlson attempted to utilize germanium in the cells to narrow the optical gap toward the optimum solar cell value of about 1.5 eV from his best fabricated solar cell material which has a band gap of 1.65-1.70 eV. (D. E. Carlson, Journal of Non Crystalline Solids, Vol. 35 and 36 (1980) pp. 707-717, given at 8th International Conference on Amorphous and Liquid Semi-Conductors, Cambridge, Mass., Aug. 27-31, 1979). However, Carlson has further reported that the addition of germanium from germane gas was unsuccessful because it causes significant reductions in all of the photovoltaic parameters of the solar cells. Carlson indicated that the degradation of photovoltaic properties indicates that defects in the energy gap are being created in the deposited films. (D. E. Carlson, Tech. Dig. 1977 IEDM, Washington, D. C., p. 214).
In the Tech. Dig. article above referenced, Carlson also reported the addition of impurity gases, such as N.sub.2 and CH.sub.4. Carlson concludes that these "have little effect on the photovoltaic properties even when they constitute 10% of the discharge atmosphere," but 30% of CH.sub.4 causes degradation of the photovoltaic properties. No suggestion is made by Carlson that the addition of these gases can increase the band gap of the resulting material. Carlson does state in the first referenced article that the development of a boron-doped "wide band gap, highly conductive p-type material" is desirable, but made no suggestion as to which of "several additives" should be utilized to open the band gap. Carlson further stated that "there is no evidence to date that the material can be made highly conductive and p-type."
After the development of the glow discharge deposition of silicon from silane gas was carried out, work was done on the sputter depositing of amorphous silicon films in an atmosphere of a mixture of argon (required by the sputtering deposition process) and molecular hydrogen, to determine the results of such molecular hydrogen on the characteristics of the deposited amorphous silicon film. This research indicated that the hydrogen acted as a compensating agent which bonded in such a way as to reduce the localized states in the energy gap. However, the degree to which the localized states in the energy gap were reduced in the sputter deposition process was much less than that achieved by the silane deposition process described above. The above described p and n dopant gases also were introduced in the sputtering process to produce p and n doped materials. These materials had a lower doping efficiency than the materials produced in the glow discharge process. Neither process produced efficient p-doped materials with sufficiently high acceptor concentrations for producing commercial p-n or p-i-n junction devices. The n-doping efficiency was below desirable acceptable commercial levels and the p-doping was particularly undesirable since it reduced the width of the band gap and increased the number of localized states in the band gap.
The non optimum spectral response of prior art amorphous silicon photoresponsive devices is overcome in accordance with the present invention by adding one or more band gap increasing elements to an amorphous photoresponsive alloy at least in one or more p doped regions thereof to adjust the band gap to an increased utilization width for particular applications.
The amorphous alloy preferably incorporates at least one density of states reducing element which can be added during deposition or thereafter. The band gap increasing element(s) can be activated and may be added in vapor deposition, sputtering or glow discharge processes. The band gap can be increased as required for a specific application by introducing the necessary amount of one or more of the increasing elements into the deposited alloy in at least one p doped region thereof. The band gap is increased without substantially increasing the number of states in the band gap of the alloy and devices, because of the presence of the reducing element in the alloy.
Since the band increasing element(s) have been tailored into the material without adding substantial deleterious states, the new alloy maintains high quality electronic qualities when the adjusting element(s) are added to tailor the wavelength threshold for a specific photoresponse application.
While the principles of this invention apply to each of the aforementioned deposition processes, for purposes of illustration herein a vapor and a plasma activated vapor deposition environment are described. A glow discharge system is disclosed in U.S. Pat. No. 4,226,898 entitled, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, issued Oct. 7, 1980 to Stanford R. Ovshinsky and Arun Madan, which is incorporated herein by reference, which system has other process variables which advantageously can be utilized with the principles of this invention .